The present invention relates to a coherent detector for demodulating a signal modulated by phase-shift keying, more particularly to a detector that operates in a slow-fading environment, does not require carrier recovery, and is suitable for large-scale integration.
Phase-shift keying (PSK) is a digital modulation system that is being increasingly adopted in mobile wireless communication systems, including microcellular automobile telephone systems and the Japanese personal handyphone system (PHS). Receiver equipment for these systems has generally employed differential detection, which is comparatively resistant to fading, but for systems operating in slow-fading environments, coherent detection has the advantage of higher sensitivity. By operating successfully at lower carrier-to-noise ratios, coherent detection can provide a greater communication range for a given transmitting power.
Conventional coherent PSK detectors employ a carrier signal recovered from and therefore synchronized with the received PSK signal, but this scheme is not without certain problems. One problem is that carrier recovery is an inherently complex process, requiring a large and expensive circuit configuration, typically one with components not suited for large-scale integration. Another problem is that the carrier recovery circuit generally includes a feedback loop with a limited response speed; this problem is particularly troublesome in communication systems that transmit in short bursts.
Attempts have been made to circumvent these problems. Some proposed systems employ differential detection to extract a tentative data signal, from which the carrier signal is generated by predictive processing. These systems avoid using a feedback loop, but the rate of differential detection errors is high, so the accuracy of carrier recovery is low, and further complex circuitry becomes necessary to protect against loss of synchronization.
Another proposed system employs a dual open loop for carrier recovery, thus using a feedforward instead of a feedback loop. Still another proposed system uses a feedback loop, but employs differential detection during initial acquisition. The carrier recovery circuits in both of these systems, however, are large and complex. There is a need for a coherent PSK detector that is not burdened with complex carrier recovery circuitry.